System for transmission and reception of frequency modulated signals



March 24, 1942 J. E. SMITH ETAL SYSTEM FOR TRANSMISSION AND RECEPTION OFFREQUENCY MODULATED SIGNALS Original Filed April 27, 1939 i uw 'vu' u."

Patented Mar. 24, 1942 SYSTEM FOR TRANSMISSION AND RE- CEPTION FFREQUENCY MODULATED SIGNALS James Ernest smith, Brooklyn, N. Y., JamesN.

Whi

taker, Weehawken, N. J., and Gilbert R..

Clark, Brooklyn, N. Y., assignors to Radio Corporation of America, acorporation of Delaware Original application April 27, 1939, Serial No.

270,332. Divided and this 1949, Serial No. 335,204

13 Claims. (Cl. P18-5) This invention is a division of our applicationled April 27, 1939, Serial No. 270,332, andrelates to the variation ofcarrier waves by change in frequency instead of amplitude. It will bedescribed in connection with photo radio transmission, although it iscapable of various other uses.

As is now well known, changes in the condition and height of theionosphere produce fading effects in radio transmission. These changesmay at times affect all frequencies within a wide range and at othertimes affect only a particular frequency or frequencies within a quitenarrow range. The rst is usually referred to as general fading and thesecond as "selective fading." I'hese two eifects may be produced atdifferent times or they may occur simultaneously.

To overcome the effects of fading ithas been proposed to modulate thecarrier wave by changing the frequency rather than the amplitude, sincefading produces amplitude variations of the signal. Frequency modulationwould thus enable one to discriminate between the signal modulation andthat produced by fading. By using frequency modulation at thetransmitter 'and rigidly limiting the voltage or current at thereceiving station, the amplitude modulation produced by fading can beeliminated. For a disclosure of this general method of reducing fading,reference is made to the patent of Wright and Smith, No. 1,964,375, June26, 1934.

Also, in producing frequency variation it has been found advantageous tovary the frequency of a sub-carrier and then amplitude modulate theradio frequency carrier by this frequencymodulated sub-carrier. Whilethe improvements produce an improvement in the transmission of picturesas compared to systems using amplitude modulation, selective fadingeffects are still noticeable. Most objectionable is the selective fadingof the radio frequency carrier. Whenever this occurs the received signalappears to be overmodulated by the side bands, thus producing spuriouscomponents inv the detected output of the side-band separation-frequencyand harmonics thereof. The main spurious component is the secondharmonic of the sub-carrier caused by this beating together of the twoside bands. Other even harmonics may be produced, but to a less extent.

application-May 15.

a radio carrier modulated by a sub-carrier frequency and its harmonic,the latter two frequencies having been modulated by the signal.

y Another object is to receive both the modulated sub-carrier frequencyand the double sub-carrier frequency and select the latter at all times.

Another object of the invention is to provide a novel arrangement forfrequency-modulating a sub-carrier frequency.

Other objects will appear in the following description, reference beinghad to the drawing, in which:

Fig. 1 is a part diagrammatic and part block illustration of atransmitting system embodying our invention. Y

Fig. 2 is a block diagram of the receiving circuit.

Fig. 4 gives voltage-frequency graphs of the frequency detector.

Referring to Fig. 1, the picture scanner may be of any desired kind asindicated at I. It has a tone output by control of a source 2 of audiooscillations. The tone output at 3 varies the frequency of a sub-carrierfrequency oscillator, which in turn modulates a radio frequency carrier.'Ihe sub-carrier frequency oscillator is of the beat or heterodyne type.The tone output 3 of the scanner feeds into double rectifier 4 and therectified output is applied to tube 5 as a negative bias that varieswith the scanner output.

Two high frequency oscillators 6 and 1 are adjusted to oscillate atfrequencies differing by a low frequency beat, for example, 800 cycles.Oscillator 6 maintains a constant frequency, but

, oscillator 'I has its frequency varied by the vac- It is one object ofour invention to overcome A the effects of selective fading by receivingand utilizing the second harmonic of the sub-carrier at all times.

Another object of the "invention is to transmit 56 uum tube stage 5.Tube 5 has its plate-cathode circuit in shunt with the high resistance 8and condenser 8' and its plate connected through the condenser 9 to thetank circuit consisting of coil I0 and condenser I0. The anode circuitof tube '5 is thus in parallel with the grid portion of the coil I0 ofthe tank circuit..

The oscillator 6 has atank circuit Illa and lll'a and is identical withoscillator 1 except for the frequency variation element shunted acrossthe lower half of the coil I0. VThe output of oscillator 6 is adjustablyconnected to control grid Il of detector tube I2, which is surrounded byscreen grid I3, The voltage on this screen is preferably lower than thaton the plate of the tube. The output of oscillator 'I is adjustablyconnected to another control grid I 4 of tube I2.

Fig. 3.is a diagrammatic illustration of the band pass frequencydetector of Fig. 2.

The output of detector tube l2 is passed through a low pass lter I5 tofilter out all frequencies except the varying beat frequency producedby\the constantv frequency of oscillator 6 and the varying frequency ofoscillator 1. Part of the low pass filter output, which consists of thefrequency modulated sub-carrier frequency, is then amplified at 20 andpassed through a band pass filter 2| into the mixing circuit 22. Thefilter 2l is constructed to pass the frequency band of the varyingsub-carrier frequency but not any sub-harmonics that may be present. Theother part of the output of filter I5 passes through the frequencydoubler 23 to produce a modulated second harmonic of the sub-carrierfrequency. The doubled frequency is then passed into mixing circuit 22through band pass filter 24, made to pass the frequency band of themodulated second harmonic. The combined output in 22 then passes intothe modulator 25 to modulate a high frequency carrier produced bygenerator 26. Transmitter 21 radiates the modulated high frequencycarrier by means of antenna 28. The modulator 25, generator 26 andtransmitter 21 may be of any type desired, as no special requirementsare imposed thereon by our improvement.

In Fig. 2 the receiving antenna 29 is connected to an appropriatereceiving apparatus 30, which may consist of radio frequency amplifiers,a detector and audio frequency amplifiers and any other desired devicesfor converting the signals into audio frequencies of the desiredintensity. The output of the amplifier 30 passes through a' band passfilter 3l and a voltage limiter 32 into a frequency detector 33 andrectifier 34 to recorder 35. The recording may be carried out by thephotographic process, visual process, or by any other method, as ourinvention is independent of the particular type of the recorder.

The band pass filter 3l is made to exclude the frequency band of thefundamental of the subcarrier frequency and to pass the band of thesecond harmonic thereof, for reasons that will appear later in theexplanation of the method of operation of our improvement.

The voltage limiter 32 limits both halves of the alternating waves andmay be of any suitable type, a number of which are well-known in theart, but we prefer the one described in the parent application abovereferred to.

The frequency detector 33 of Fig. 2 is shown in detail in Fig. 3. Theinput lines 36 introduce the signal into a low pass filter throughtransformer 31. It is essentially a low pass filter consisting of seriesinductances 38, 39 and shunt capacities 40, 4I and 42, arranged in theusual way. These inductances and capacities are of such number and valuethat the characteristic curve A shown in Fig. 4 will have asubstantially straight portion a, b between the frequencies f1 and lz.The modulation frequencies f1 and fz may have various values, but inpractice we have found it satisfactory to use a sub-carrier frequencyvarying between 800 and 1,000 cycles per second for the fundamental,which, when doubled by the frequency doubler, would have aV sited, andthen the characteristic will be like curve B in Fig. 4.

The operation will now be described:

The tone output of scanner I is rectified by double rectifier 4 and therectified potential varies the plate-cathode impedance of tube 5 andthus varies the tuning effect of condenser 9 in oscillator 1. By varyingthe eective impedance (anode-cathode impedance) in series with condenser9, the impedance of the shunt around the lower half of coil l0 is variedand this varies the frequency of the tank circuit, which is thefrequency producing element in the oscillator.

Oscillator 6 is set to oscillate at some suitable frequency, forexample, 75 kilocycles, and oscillator 1 will oscillate under control ofthe scanner at a frequency that'varies above or below the frequency ofoscillator 6 a suflicient amount to produce a beat, the latter varyingfrom, say, 800 to 1,000 cycles. This beat or sub-carrier frequency isproduced in the output of detector tube I2 by connecting the output ofoscillators 6 and 1 to control grids II and I4, respectively. Low passfilter I5 permits the varying sub-carrier frequency to pass and thehigher frequencies are filtered out.

A part of the fundamental band (800 to 1,000 cycles) of th'e sub-carrierfrequency, passes through amplifier 20 and band pass filter 2| intomixing circuit 22 and a part has the frequency doubled to 1,600 to 2,000cycles before it passes into the mixing circuit 23 through band passfliter 24. The combined fundamental and second harmonic frequencies thusmodulate the high frequency carrier from 26 and the Wave transmittedfrom the antenna has side bands produced by the fundamental and sidebands produced by the second harmonic, both of which contain thefrequency modulation produced by the output of scanner I.

The radio frequency carrier will be received by antenna 29 and receivingapparatus 30 of Fig. 2. It will be amplified and detected and theresultant signal amplified at audio frequency in the usual way. 'Theoutput of the receiver, when there has been no fading of radio frequencycarrier, will contain both the varying sub-carrier frequency and thevarying doubled frequency thereof. The former is blocked by the bandpass filter 3|, but the latter passes readily through. Due to the effectof the upper atmosphere, the signal will have spurious amplitudemodulation in most cases. When the signal passes through limiter 32, asufficient portion of the positive and negative tops of the alternatingwave will be cut off to eliminate this spurious amplitude modulation andthe signal will pass into the frequency detector 33 free from thisaddition, but with all the frequency modulation produced by the scannerI.

On the straight portion a-b of the curve of filter 33, the output variesin amplitude directly with the frequency. Hence filter 33 is a frequencydetector that converts the frequency modulation of the sub-carriersecond harmonic into amplitude modulation whichis rectified at 34 andrecorded at 35.

It will thus be seen that, normally, that is, with no selective fading,the transmitted second harmonic of the sub-carrier frequency isreceived, limited, converted and recorded and the transmittedfundamental of the sub-carrier frequency is not utilized.

When the radio frequency carrier selectively fades, the transmittedharmonic of the subcarrier frequency will be doubled by the beating ofthe side bands and it will not pass through the band pass filter 3|. Thefundamental of the sub-carrier frequency will likewise be doubled by asimilar beating action. Hence it will be passed by the filter 3l andwill reach the recorder in a way that will be understood from theprevious description.

Briefly, with our invention, the receiving station utilizes the secondharmonic produced at the transmitting station when there is no selectivefading and utilizes the second harmonic formed by the beating of theside bands when there is selective fading.

The conversion of the frequency modulation into amplitude modulation canobviously be accomplished by means other than low or high pass filters,as other devices are known in the art.

The frequency modulation arrangement in our invention may, of course, beused to transmit only the modulation produced by the sub-carrierfundamental for reception by the receiving apparatus of our parentapplication, by appropriate disconnection of the frequency doubler 23and its associated apparatus.

Various modifications of our invention may be made without departingfrom the spirit of the invention.

Having described our invention, what we claim 1s:

1. In a signaling system, means for producing a sub-carrier frequency,means for varying said sub-carrier frequency in accordance with asignal, means for producing a second harmonic of said varyingsub-carrier frequency, means for producing a radio frequency carrier,means for modulating the radio frequency carrier by both saidsub-carrier frequency and said second harmonic, means for transmittingthe modulated radio frequency carrier, and means at the receiverforutilizing the second harmonic only of the sub-carrier frequency andrejecting the fundamental thereof.

2. In a signaling system, means for producing a sub-carrier frequency,means for varying the said sub-carrier frequency in accordance with asignal, means for producing a second harmonic of said varyingsub-carrier frequency, means for producing a radio frequency carrier,means for modulating the radio frequency carrier by both saidsub-carrier frequency and said second harmonic, means for transmittingthe modulated radio frequency carrier, a receiver having means fordetecting the second harmonic only of the sub-carrier frequency andmeans for limiting the amplitude of the detected frequency.

3. In a signaling system, means for producing a sub-carrier frequency,means for varying said .sub-carrier frequency in accordance with asignal, means for producing a second harmonic of said varyingsub-carrier frequency, means for producing a radio frequency carrier,means for modulating thel radio frequency carrier by both saidsub-carrier frequency and said second harmonic, means for transmittingthe radio frequency carrier, a receiver having means to detect thefundamental and the second harmonic of the sub-carrier frequency andmeans for selecting the second harmonic frequency only.

4. In a signaling system, means for producing a sub-carrier frequency,means for varying the said sub-carrier frequency in accordance with asignal, means for producing a second harmonic of said varyingsub-carrier frequency, means for producing a radio frequency carrier,means for 5. In a signaling ystem, means for producing a sub-carrierfrequency, means for varying said sub-carrier frequency in accordancewith a signal, means\for producing a second harmonic of said varyingsub-carrier frequency, means for producing a radio frequency carrier,means for modulating the radio frequency carrier by both saidsub-carrier frequency and said second harmonic, means for transmittingthe modulated radio frequency carrier, a receiver having means to detectthe fundamental and the second harmonic of the subcarrier frequency,means for selecting the second harmonic frequency and means for limitingthe amplitude of the selected frequency.

6. In a. signaling system, means for producing a sub-carrier frequency,means for varying the said sub-carrier frequency in accordance with asignal, means for producing a second harmonic of said Varyingsub-carrier frequency, means for producing a radio frequency carrier,means for modulating the radio frequency carrier by both saidsub-carrier frequency and said second harmonic, means for transmittingthe modulated radio frequency carrier, a receiver having means to detectthe fundamental and the second harmonic of the sub-carrier frequency,means for selecting the second harmonic frequency, means for limitingthe amplitude of the selected frequency, and meansfor converting thefrequency e modulation of the limited frequency into amplitudemodulation.

7. A transmitting system comprising a source of modulating potential, asource of constant al-l ternating potential', means for modulating saidalternating potential by said modulating potential, means for rectifyingthe modulated alternating potential, means for producing a carrierfrequency, means for producing a sub-carrier frequency, means forvarying the sub-carrier frequency by the output of said rectifyingmeans, and means for modulating the carrier frequency by the varyingsub-carrier frequency.

8. A transmitting system comprising a source of modulating potential, asource of constant low frequency alternating potential, means formodulating said low frequency alternating potential by said modulatingpotential, means for rectifying the modulated alternating potential,means for producing a. carrier frequency, vmeans for producing asub-carrier frequency, means for varying the sub-carrier frequency bythe output of said rectifying means, and means for modulating thecarrier frequency by-the varying subcarrier frequency. A

9. A transmitting system comprising a source of modulating potential, a.source of constant low frequency alternating potential, means formodulating said low frequency alternating potential by said modulatingpotential, means for rectifying the modulated alternating potential,

means for producing a carrier frequency, means and means for amplitudemodulating the carrierl frequency by the varying sub-carrier frequency.

10. A transmitting system comprising a source of modulating potential, asource of constant low frequency alternating potential, means formodulating said low frequency alternating potential by said modulatingpotential, means for rectifying the modulated alternating potential,means for producing a carrier frequency, means for producing asub-carrier frequency, means for varying the sub-carrier frequency bythe output of said rectifying means, means for amplitude modulating thecarrier frequency by the varying sub-carrier frequency, and means fortransmitting a radio wave having the modulated carrier frequency.

1l. A transmitting system comprising means for producing a varyingoutput potential, a generator of low frequency alternating potential,means for varying the amplitude of said alternating potential inaccordance with said varying output potential, means for rectifying themodulated alternating potential, means for generating a high frequencycarrier potential, means for generating a constant intermediatefrequency potential, means for generating a second intermediatefrequency potential, means for varying the frequency of thesecond-mentioned intermediate frequency potential with the amplitude ofthe potential of the output of said rectifying means, means for beatingtogether the outputs of the two intermediate frequency generators, andmeans for modulating the high frequency carrier potential by the beatfrequency potential of said two intermediate frequency generators.

12. A transmitting system comprising a picture scanner producing avarying output potential, a generator of low frequency alternatingpotential, means for varying the amplitude of said alternating potentialin accordance with the varying output potential of said scanner, meansfor rectifying the modulated alternating potential, means for generatinga high frequency carrier potential, means for generating a constantintermediate frequency potential, means for varying the frequency ofthe'second-mentioned intermediate frequency potential with the amplitudeof the potential of the output of said rectifying means, means forbeating together the outputs of the two intermediate frequencygenerators, and means for modulating the high frequency carrierpotential by the beat frequency potential of said two intermediatefrequency generators.

13. A transmitting system comprising a picture scanner producing avarying output potential, a generator of low frequency alternatingpotential, means for varying the amplitude of said alternating potentialin accordance with the varying output potential of said scanner, meansfor rectifying the modulated alternating potential, means for generatinga high frequency carrier potential, means for generating a constantintermediate frequency potential, means for generating a secondintermediate frequency potential, means for varying the frequency of thesecond-mentioned intermediate frequency potential with the amplitude ofthe potential of the output of said rectifying means, means for beatingtogether the outputs of the two intermediate frequency generators, andmeans for amplitude modulating the high frequency carrier potential bythe beat frequency potential of said two intermediate frequencygenerators.

JAMES ERNEST SNIITH. JAMES N. WHITAKER. GILBERT R. CLARK.

